When drilling for hydrocarbons such as oil and gas in a subterranean formation, a wellbore is drilled into the formation for the purpose of gaining access to the hydrocarbons. Drilling mud is often used to assist in the drilling process and to hydrostatically suppress the flow of fluids into the well from a pressurized formation. A casing is installed and cemented to the face of the wellbore. During this process the drilling mud is, for the most part, replaced with water. The casing blocks the flow of fluid from the formation into the wellbore. Charges are then detonated so as to cause perforations in the well casing and cement. These perforations extend into the formation and permit the influx of the hydrocarbons into the well. The perforation is typically carried out using a perforating gun.
Perforation can be conducted in "overbalanced" conditions where the well pressure is greater than the pressure at which the formation will fracture. The term "underbalance" is used to refer to a situation where the pressure of liquid in the wellbore is less than the pressure existing in the formation.
After the perforations have been created, the drilling mud is gradually displaced by the influx of hydrocarbons and the hydrocarbons eventually flow into the wellbore and then up to the surface.
Conventional drilling methods and assemblies suffer from certain disadvantages. For example, it can be problematic to clean the perforations of mud filtrate, cement contaminates and perforation debris. In standard extreme overbalance perforating applications it is not possible to surge the well prior to flowing the well back to surface.
These and other deficiencies in the prior art have been addressed by the present invention.